The invention relates to a device for carrying out spectroscopic measurements by the principle of interferometry, with a matrix of photodetectors.
The current state of spectroscopy includes methods operating on the dispersive and the interferometric principles.
In the dispersive method, various dispersion agents, such as prisms and diffraction gratings (scratched or holographically etched) and in some cases wedge filters are used. The key element of this method is a spectrograph or a monochromator. A spectrograph is designed so that the entire wavelength range to be measured is detected in one step, and the complete spectrum is shown at the output. The spectrum is recorded with a photographic film, video camera, reticon or a CCD (charge coupled device). In the monochromator, light of only one wavelength at a time is delivered to the output. In order to pick up a spectrum, the prism or reflection grating is rotated, so that all the wavelengths are admitted in succession over time. For recording, a photodiode, photomultiplier, or bolometer is typically used.
In the interference method, a Michelson interferometer is often used, in which the light to be measured is carried through two partially transparent mirrors, and the spacing of the mirrors from one another is varied in order to move all the wavelengths through. The recording of the light at the output is done by means of a photodiode. The resultant course over time of the output signal of the photodiode is then subjected to a Fourier, Hadamard or similar transform, in order to obtain the spectrum.
A modified Michelson interferometer is disclosed in WO 85/03122, which makes do without a moving mirror. This is attained in that one of the mirrors is tilted, so that a two dimensional interference pattern is produced. This interference pattern is read out by means of a photodetector matrix.
From German Patent Application DE 39 28 001 A1, a multi-beam interferometer for distance measurement is known, having a monochromatic light source and one fixed and one movable mirror. One of the two mirrors has a stepped profile.
Monochromators, spectrographs and interferometers are as a rule large, expensive, and heavy and usually includes parts that have to be moved mechanically, which always entails problems with respect to accuracy. The efficiency of dispersive elements is moreover very greatly dependent on the wavelength.